Japanese utility model laid open publication No. 3-17362 discloses a typical conventional filter device for use in automotive automatic transmission systems. Such a filter device is typically provided in a conduit leading to an inlet of an oil pump, and is required to be placed in a lower part of the oil sump of the automatic transmission system to ensure the working fluid to be always supplied to the inlet end of the oil pump even when the vehicle is making a turn at high speed or travelling on steep slopes. Therefore, the filter device is required to be constructed as a highly compact unit, and it is often necessary to provide an inlet and an outlet of the filter device close to each other. As a result, the flow of the working fluid may not be favorably guided inside the filter device so that high flow speed regions and stagnant regions may be created inside the casing of the filter device. This in turn causes the flow of the working fluid to be distributed unevenly over the effective surface area of the filtering material so that the entire filtering material may not be effectively utilized, and the pressure drop across the filter device may be undesirably increased.